So maybe American 180 style where the pan rotates, but the bottom with the feed hole is fixed? Maybe take a look at the American 180 AR-15 upper to get some easier ideas? Trying to replicate the lewis might be more work replicating bullet feeds, feed arm lever system and indexing of the pan with the pawls. There are easier methods. If the bottom is fixed I think the indexing will be much easier and loading could be easier too? Maybe lexan so you can see what's going on inside? Only one layer.......minimal fun.
British guns allowed you to move the charging handle from one side to the other, the slide down plates are just debris covers for the slots and a safety. Only one handle will only fit but can be switched. The US made Savage Lewis does not have a slot for a handle on the other side, the receiver is closed off completely.
An ambitious project, best of luck reinventing the wheel LOL